1. Field of the Invention
The present invention relates to a vibration wave driven motor, and, more particularly, to a frictional material used so as to form a friction surface of the vibration wave driven motor.
2. Related Background Art
Hitherto, there has been disclosed a frictional material of a friction member of a vibration wave driven motor in, for example, Japanese Patent Laid-Open No. 62-100178. According to this disclosure, a WC-Co film is used as the frictional material on the oscillator side of the vibration wave driven motor, while a hard alumite is used as the frictional material on the moving side of the same so that the durability of the motor is improved.
However, both the above-described conventional frictional materials WC-Co and hard alumite are brittle materials, having, poor damping performance. Therefore, they may be easily worn and encounter problems in terms of noise. It is known that the WC-Co material has a large friction coefficient expressed by .mu. because of a reason to be described later It is also known that hard alumite has a large friction coefficient expressed by .mu.. Therefore, there has been a technology in which molybdenum disulfide is impregnated into openings formed in the alumite film in order to reduce the friction coefficient expressed by .mu. so as to use it as the moving portion of a bearing or the like which must have a satisfactory sliding performance. As described above, it has been necessary to select a suitable material so as to meet the specification of the vibration wave driven motor from the conventional frictional materials although they respectively have a too large friction coefficient. For example, the vibration wave driven motor employing a material formed by combining the WC-Co film and the hard alumite as the frictional material thereof has been employed in a lens moving mechanism for realizing the automatic focusing function of a camera. In this case, the life of the vibration wave driven motor for a camera of this type is allowed to be shorter than that of electromagnetic motors. Furthermore, the quantity of wear in the material manufactured by combining the above-described frictional materials is able to satisfactorily meet the specification in the case of the vibration wave driven motor for a camera. As for noise, it can be considerably prevented by reducing the surface roughness so as to maintain a uniform contact state. Since the above-described frictional material is a porous material, the driving force can be extremely deteriorated if a small quantity of oil adheres to its surface. However, since the above-described vibration wave driven motor for the camera lens is arranged in such a manner that the overall body of the motor is substantially hermetically sealed, no problem arises. However, the oscillator motor of this type encounters a problem in terms of the performance when it is employed in another technical field. That is, a frictional material exhibiting excellent driving force transmission, wear resistance and noise eliminating performance has been desired.